Solar Energy

Fundamental Considerations

Electromagnetic Energy

The term electromagnetic energy comprises all types of energy that
travels from its source through space in the form of harmonic waves
along straight paths at the uniform speed of light (3x108
m/sec). Radiation is the term that pertains to the emission and
propagation of electromagnetic energy in the form of waves.

There are many types of electromagnetic energy, but consideration of
the subject is necessarily limited to those of solar origin that
provide pertinent background information for the proper utilization of
solar radiation for disinfection purposes. It should be recognized
that solar radiation constitutes only a portion of the entire
electromagnetic energy spectrum. In fact, sunlight as sensed by the
human eye essentially represents that part which is visible, although
it also includes other invisible radiation components. Strictly
speaking, the terms light and sunlight refer to
radiation wavelengths detectable by the human eye (400 to 700 nm).

Electromagnetic radiation, as well as solar radiation, is commonly
classified on the basis of radiation wavelength into several regions,
or bands. The wavelength bands of solar radiation, both visible and
invisible, are mentioned in Table 1 along with some useful remarks.
Similar information is also illustrated diagrammatically in Figure 1
for purposes of visual clarification. Note that the colours shown for
the various wavelength bands in the visible region of the solar
radiation spectrum are approximate representations of the colours of
light within each band.

Band

Wavelength (nm)

Atmospheric Effects

Gamma ray

<0.03

Completely absorbed by the upper atmosphere

X-Ray

0.03 - 3

Completely absorbed by the upper atmosphere

Ultraviolet, UV

UV (B)

3 - 300

Completely absorbed by oxygen, nitrogen, and ozone in the upper
atmosphere

UV (A)

300 - 400

Transmitted through the atmosphere, but atmospheric scattering
is severe

Visible

400 - 700

Transmitted through the atmosphere, with moderate scattering of
the shorter waves

Infrared, IR

Reflected IR

700 - 3000

Mostly reflected radiation

Thermal IR

3000 - 14000

Absorption at specific wavelengths by carbon dioxide, ozone, and
water vapour, with two major atmospheric windows

Propagation of Solar Energy

The sun continuously radiates enormous amounts of solar energy at
wavelengths that cover the ultraviolet, visible, and infrared bands. The
maximum intensity of the emitted solar energy occurs at a wavelength of
about 555 nm, which falls within the band of green light (Figure 1).

Solar radiation moves freely in outer space because of the vacuum,
unless its path is obstructed by planets, satellites, meteorites, or
other space objects. Whatever portion of it reaches the earth and its
surrounding atmosphere may encounter a variety of atmospheric or
terrestrial objects. When solar radiation strikes any object whether
in the form of solid, liquid or gas, changes in its magnitude,
direction, and wavelength are expected to occur depending upon the
nature and characteristics of the intervening object. These changes
may come as a result of any of the following possible phenomena:

Radiation may be transmitted through a transparent
object such as air, water, or glass with a change in speed and
direction.

Radiation may be partially or completely absorbed by an
object, the components thus absorbed being dependent on the wavelength
of the specific radiation and the characteristics of the object. Blue
tinted glass, for instance, would transmit radiation with wavelengths
of 425 to 490 nm and, at the same time, absorb radiation in the other
wavelength bands.

Radiation may be scattered by being deflected in all
directions, a common example being the scattering of sunlight as it
traverses the atmosphere.

Radiation may be reflected by being returned from the
surface of an object in an unchanged form except for the deviation
whereby the angle of reflection would be equal and opposite to the
angle of incidence.